Use of AZXX Mg alloys is widespread as they are ideal alternatives to Al alloys and steel due to their lightweight nature and the corresponding mechanical properties (e.g. specific stiffness, specific strength). Among the AZXX series, AZ31, AZ61 and AZ91 are the major commercial alloys. However, the high chemical reactivity and poor corrosion resistance of Mg alloys have limited their use. In general, increasing Al content in AZXX alloys leads to better corrosion resistance but decreases ductility due to the formation of the brittle Mg17Al12 intermetallic phase (β). In particular, AZ91 has been extensively investigated due to its higher yield and ultimate tensile strengths as well as better corrosion resistance than AZ31 and AZ61. In comparison, AZ31 can be used to form more complex shapes due to better ductility, but has high chemical reactivity associated with the low Al content of about 3 weight percent (wt. %).
Chemical conversion coatings (CCs) are widely used as the initial layer of a coating system for protection of Mg alloys. Cerium-based conversion coatings (CeCCs) are capable of providing excellent corrosion resistance for high strength Mg and Al alloys when proper processing parameters are used.
Bulk CeO2 has a stable cubic structure (fluorite type, space group Fm3m) from room temperature to the melting point (about 2500° C.). CeO2-x has a cubic fluorite structure up to x≈0.2 but additional structures such as rhombohedral, monoclinic, and triclinic are possible at 0.2<x<0.3. The electronic structure of cerium gives its compounds unusual physical, chemical and electrochemical properties. Cerium exists in two oxidation states, Ce(III) when the 4f orbital is occupied with one electron (4f′) and Ce(IV) when unoccupied (4f0).
Cerium based oxides, such as oxygen deficient CeO2-x, are technologically important because the Ce(III)/Ce(IV) couple may undergo rapid reduction-oxidation (redox) cycles at particular environmental conditions. The reduction mechanism from Ce(IV) to Ce(III) species in cerium oxides is not known, but Ce(III) is favored in oxygen-deficient atmospheres at elevated temperatures (e.g., 200-1000° C.). Thus, a dramatic alteration of environmental conditions is often necessary to effect a Ce(IV)/Ce(III) redox cycle.